1. Field of the Invention
The present invention relates to a device for measuring semiconductor characteristics relying upon a photovoltaic method.
2. Description of the Prior Art
The photovoltaic method has long been employed in the field of semiconductor measurements, owing to its advantage as a non-contact measuring method over, for example, the four-point-probe method for measuring resistivity. FIG. 1 is a diagram for illustrating the fundamental principle of a conventional method of measuring resistivity distribution of semiconductor specimens utilizing a photon beam.
When a surface 2' of a semiconductor specimen 2, which spreads in a two-dimensional manner, is irradiated with a photon beam 1, electron-hole pairs consisting of holes 3 and electrons 4 usually develop on the surface 2' of the specimen 2, and diffuse toward the back surface 2" of the specimen 2 as indicated by arrows 3', 4'. In the case of silicon, as is well known, however, the electrons 4 have greater mobility than that of the holes 3. In other words, the electrons 4 move in larger number than the holes 3 toward the back surface 2". Therefore, the holes 3, having positive charge, are left in large amounts on the surface 2' of the semiconductor specimen 2 and, consequently, the surface 2' of the specimen 2 is positively charged. This phenomenon was reported in 1931 by H. Dember of Germany, and has, since then, been known as the Dember effect. The voltage produced by the Dember effect, i.e., the Dember voltage, however, is much smaller than the voltage that develops when the p-n junction is irradiated with light, and has not heretofore been utilized for any specific purposes.
The inventors of the present invention have found that the following result is obtained from the n-type wafers, such as those formed of silicon, ##EQU1## where .DELTA.V.sub.D denotes a Dember voltage, and each of the symbols has the following meaning:
b: mobility of electrons/mobility of holes,
S: area of wafer,
.rho.(0): resistivity of the wafer surface,
e: electric charge of the electron,
I: intensity of the photon beam (photon flux/sec),
.alpha.: photon beam absorption coefficient,
L.sub.p : diffusion length for minority carriers,
V.sub.p : diffusion velocity for minority carriers
S.sub.f : recombination velocity of carriers on the wafer surface.
As is obvious from the equation (1) above, the Dember voltage is dependent upon many factors. If all of the factors except for the resistivity .rho.(0) are regarded as being constant, the above equation can be written as, EQU .DELTA.V.sub.D =K.multidot..rho.(0) (2)
where K is a constant.
Namely, if the semiconductor specimen (wafer) 2 without the junction is scanned by converging the photon beam 1 and if the distribution of photovoltage at that time is measured, the measured result is a Dember voltage, which, finally, is equal to the measurement of resistivity distribution on the surface of the specimen 2.
A Schottky junction has heretofore been used to detect the distribution of resistivity. FIG. 2 illustrates a fundamental principle thereof. An ohmic electrode 6 is attached to the back surface 2" of the semiconductor specimen 2, a metal probe 5 is erected on the surface of the specimen 2, and the vicinity of the probe 5 is irradiated with the photon beam 1. As is well known, a photovoltage develops in the Schottky junction 5' and is measured by a voltmeter 7. Usually, the intensity of the photovoltage depends upon the resistivity of the portion of the specimen 2 to which the metal probe 5 is opposed. Therefore, the indication of the voltmeter 7 varies in proportion to the resistivity. As for the surface 2 of a wide wafer, the metal probe 5 needs to be simply moved. In practice, however, this operation is not practical. As shown in FIG. 3, therefore, mesh electrodes 8 are pressed with pressure onto the specimen 2 to form a Schottky junction 8" on the whole surface. By scanning the specimen 2 with the photon beam 1, it is possible to detect the distribution of resistivity on the surface 2'.
The conventional method shown in FIG. 3, however, has defects. First, characteristics of the Schottky junction 8" depend on the mechanical pressure of the metal, surface conditions of the metal (roughness, oxide layer, etc.), and surface conditions of the semiconductor (oxide layer, humidity, dust, etc.), which make it difficult to form a uniform junction over wide areas. Second, portions of the surface are covered with mesh electrodes 8, so the whole surface of the specimen 2 is not irradiated with the photon beam 1. Third, attachment of the ohmic electrode 6 damages the specimen 2, and makes it difficult to carry out a perfect non-destructive insepection.
Measuring the characteristics of the specimen 2 by forming a Schottky junction using an electrolyte 13 such as Na.sub.2 SO.sub.4 as one electrode as shown in FIG. 4, based upon the same principle as the method of FIG. 3 has also been reported. The electrolyte 13, however, involves clumsy operation if it is attempted to use it as a transparent electrode. Further, the ohmic electrode 6 must be attached onto the back surface, as in the above-mentioned prior art. In FIG. 4, reference numeral 12 denotes an electrode, and 14 denotes a side wall of a vessel for storing the electrolyte.
As mentioned above, there has not heretofore been known any method of photovotaically measuring the resistivity distribution of the surface of the silicon wafer without damaging the specimen being measured.